Volatile chemicals stimulate chemosensory receptors in tephritid flies when assessing a potential novel host and trying to expand . Therefore, chemosensory-related genes are involved in the initial process of host plant expansion for tephritids. Olfactory-related genes of tephritids are one type of chemosensory gene that includes several gene families of odorant-binding proteins , chemosensory proteins , odorant receptors , ionotropic receptors , and sensory neuron membrane proteins , which are primarily involved in the identification of volatile chemicals, including volatiles of host fruits. After receiving odor chemical signals, these olfactory-related genes are triggered to transduce cascades that send information to specific regions of the brain, which ultimately leads to specific behavioral responses . OBP genes play an important role in the first step of chemosensory identification of insects, including tephritids . OBP genes direct odorant-binding proteins to bind volatile odor molecules specifically by distinct expression to related olfactory receptors that are bound to olfactory receptor neurons in antennae . CSP genes are regarded as playing a similar role as OBP genes involved in the initial process of chemosensory signal transmission to corresponding receptors . OBP and CSP genes are major gene types that lead tephritid flies to respond to different chemosensory chemicals, including volatile chemicals of host plants . Except for these two categories of genes, large plastic growing pots some odor receptor genes also play important roles in host odor recognition of tephritids, such as genes related to odor receptors and ionotropic receptors .
Odorant receptors of insects are composed of at least two proteins: a conserved coreceptor as an ion channel and a specific OR subunit , which determines the ligand specificity and forms structurally ligand-gated ion channels . The OR genes mediate odorant receptors of insects transmitting the odorant molecules they receive into electric signals that are transmitted to a higher-order neural center . IR genes are related to ionotropic glutamate receptors , which are regarded as ion channels . They also play important roles in odor chemical perception . The sensory neuron membrane proteins gene encodes transmembrane domain-containing proteins that belong to a large gene family of CD36 receptors . SNMPs regulates the corresponding proteins to identify chemosensory signals, mainly pheromone chemicals . The GR family is another type of chemosensory protein that is a ligand-gated ion channel broadly expressed in gustatory receptor neurons in taste organs and is mainly involved in taste recognition of CO2 , sugar, and bitterness . When receiving taste signals, GR genes are involved in identifying taste and ingestion. Among tephritid flies, Bactrocera dorsalis and Ceratitis capitata are well-known polyphagous species that have expanded their host plants to more than 250 species . However, Bactrocera minax and Z. cucurbitae are oligophagous species that mainly attack citrus fruits and cucurbit plants, respectively. Bactrocera oleae , Procecidochares utilis , and Carpomya vesuviana are monophagous species infesting olive , crofton weed , and jujube , respectively, and all have limited host plant species . Compared to several major olfactory-related gene families, the two polyphagous species have more genes, with 3 CSPs, 35 OBPs, 74 ORs, and 40 IRs in B. dorsalis and 45 OBPs, 76 ORs, and 70 IRs in C. capitata , than two host-limted species .
A similar situation was observed in the GR family. There are also more GR genes in C. capitata and B. dorsalis than in the host-limited species P. utilis , C. vesuviana , and Z. cucurbitae . The increased numbers of these genes are associated with chemosensory-related gene family expansion via gene duplication and differentiation , which exertimportant roles in tephritid fly adaptation to other hosts and expansion of their host ranges. Obvious chemosensory-related gene expansions were also reported in Tribolium castaneum , Spodoptera frugiperda , and Heliconius melpomene . For example, the pea aphid Acyrthosiphon pisum , with broader host ranges, experienced obvious expansion of the OR, OBP, and GR gene families, with 87 ORs, 18 OBPs, and 78 GRs, compared to the soybean aphid Aphis glycines , with 47 ORs, 10 OBPs, and 61 GRs . Altering gene expression levels also helps tephritids respond to different host plants and realize host expansion. OR13a and OR82 expression are higher in antennae in B. dorsalis in response to 1-octen-3-ol and geranyl acetate, respectively, which are major volatile components of its host fruits, mango and almond fruit . For B. minax, increasing the expression levels of several GR genes regulate the taste process in response to different chemosensory stimuli of hosts .Once a tephritid adult identifies a potential novel host fruit for oviposition or feeding, the plant fruit must be suitable for larval development, which includes overcoming any secondary toxic chemicals in the novel host fruit . Therefore, detoxifcationand other digestion-related genes also play core roles in mediating the host plant expansion of tephritids. Common detoxifcation-related genes of insects include gene families of cytochrome P450s , glutathione S-transferases , UDP-glycosyltransferases , carboxyl/cholinesterases and ATP binding cassettes .
The cytochrome P450 family belonging to phase I enzymes includes various CYP subfamilies for different tephritid species . The GST superfamily consists of phase II enzymes divided into at least seven major subclasses: the delta, epsilon, omega, sigma, theta, zeta, and microsomal classes . The PGE phase II enzymes are a large family that can be divided into 13 clades, including the dietary detoxification class , the hormone/semiochemical processing group , and the neurodevelopmental group . The ABC transporter superfamily belonging to phase III enzymes can be subdivided into eight subfamilies, from ABC-A to ABC-H. The cytochrome p450 gene family of phase I mainly contributes to the catalysis of numerous oxidative reactions during endogenous and exogenous metabolism . The important roles of genes in this family are the metabolism of xenobiotics, plant allelochemicals , and even insecticides. GSTs are multifunctional genes of phase II enzymes that play a crucial role in the detoxification of endogenous and xenobiotic compounds, including plant secondary metabolites and pesticides. CCE families of phase II have been shown to be involved in the detoxification of plant-derived allelochemicals as well as insecticides . The ABC transporter genes of phase III encoding membrane-bound proteins typically function in the ATP-dependent transport of various substrates across biological membranes . The roles of ABC genes are mainly in handling xenobiotics such as plant phytotoxins and insecticides . These genes can participate in regulating detoxification of host plant secondary metabolites of tephritid flies by coding corresponding enzymes, which help to transform toxins entering the insect system into hydrophilic compounds that can be eliminated and in the adaptability of different hosts . The major digestive-related genes include gene families of cysteine proteases, proteases, lipase, glucosidase, and serine proteases . The serine proteases are members of the supergene family, including chymotrypsin, trypsin, thrombin, subtilisin, plasmin, and elastase. subclasses . Various digestive proteases exert important roles in the nutrition digestion of tephritid flies from novel host plants that they try to expand to. However, protease inhibitors of host plants are a widespread defense against herbivores such as tephritids. Therefore, genes coding various proteases react to protease inhibitors by regulating inhibitor-sensitive proteases or expressing proteases that are not targets of the inhibitors . When expanding to other novel hosts, large plastic pots tephritid flies must adapt to different chemical environments from their native hosts. Detoxification-related genes regulate the host expansion of tephritids via gene family expansion similar to chemosensory-related genes. The major gene families of detoxifcation GSTs, P450s, CCEs and ABC transporters are more numerous in polyphagous B. dorsalis and C. capitata than monophagous P. utilis and B. oleae . However, reports about digestive gene family expansion in tephritids are still rare. Overall, detoxifcation and the digestive-related gene family combined with chemosensory-related gene family amplifcation exhibit a close association with host range extension. This gene family expansion is helpful for the host plant expansion of fruit flies. Cases in other insects strengthen this idea. For example, Helicoverpa armigera and Helicoverpa zea are two species of caterpillars that have considerably broader host ranges than any other lepidopterans.
Great expansion of detoxification and digestive gene families was found in the two species. In addition to gene family amplification, detoxification and digestive genes also regulate host expansion of tephritid flies by activating various gene subfamilies, subclasses, or clades. To respond to various toxic environments, including secondary toxic chemicals of different hosts, B. dorsalis primarily triggered the delta subfamily of GSTs, CYP3 and CYP4 subclasses of P450s, A–C clades of CCEs, and ABC-A, ABC-B, and ABC-G subclasses of ABC transporters , C. ceratitis activated the epsilon subfamily of GST, CYP6 and CYP12 of P450s, B clade of CCEs , and P. utilis mainly triggered the delta, epsilon and microsomal subfamilies of GSTs, CYP4, and CYP9 of P450s, C clade of CCEs and ABC-G subclass of ABC transporters , but R. pomonella mainly launched CYP4 and CYP6 of P450s . For the digestive gene family, B. dorsalis and C. capitata primarily triggered aminopeptidase, trypsin and serine peptidase digestive genes, but B. oleae, which is a strictly monophagous species, triggered serine protease and nuclease digestive genes to respond to different host secondary chemical environments . Detoxifcation- or digestion-related genes also facilitate tephritid fly adaptation to different hosts by altering gene expression levels. Rhagoletis zephyria evolved fromRhagoletis pomonella and experienced host expansion from apple to snowberry plants . Increased expression levels were found in some detoxifcation-related genes, including cytochrome P450, glutathione S-transferases, and glycosyltransferase, in R. zephyria facing the apple host environment . Z. cucurbitae is the species that mainly attacks cucurbit plants, and the fly responds to different secondary chemical environments of Mucuna pruriens plants by reducing the expression levels of trypsin and chymotrypsin digestive genes .Although the importance of chemical stimuli is highly emphasized in the host expansion of tephritids, other nonchemical stimuli, such as the color of the novel host fruit, should not be ignored. Many insects locate their host plants primarily by color signals, including beetles, Altica engstroemi , Hylastes ater , and Arhopalus ferus . For tephritid flies, Neoceratitis cyanescens , B. minax , B. dorsalis , and Z. cucurbitae are typical examples of species that appear to select different hosts frst by fruit color rather than chemical signals. Z. cucurbitae realized its host expansion to a novel host, papaya , in Hawaii by strongly relying on the color location of fruits by vision . However, the underlyinggenetic mechanism has not been revealed for Z. cucurbitae. In fact, nonchemical stimuli, such as color, are associated with vision-related genes that allow the identifcation of different hosts . The genes responsible for color discrimination in Diptera are primarily related to opsin proteins in the photoreceptor cells of the eye . Six types of Rh opsin-expressed genes have been identifed as major genes involved in color recognition and photoreception in Diptera insects. The Rh1 and Rh2 opsin genes are associated with motion detection and direction, respectively . Rh3 and Rh4 are UV-sensitive opsin genes, Rh5 is a blue-sensitive gene and Rh6 is a green opsin gene . These opsin genes lead the photoreceptor of eyes to receive various chromophore pigments and then activate a series of visual transduction cascades to launch corresponding color identifcation behavior. In the genome of polyphagous C. capitata, the long wavelengthsensitive genes Rh1, Rh2, and Rh6 and the UV-sensitive genes Rh3 and Rh4 were found, while Rh2-4 and Rh6 were found in the phototransduction pathway of oligophagous B. minax . Moreover, the role of Rh6 in modulating green color discrimination was reported in C. capitata and B. minax . In B. minax, the function of Rh6, which is responsible for green spectral sensitivity, has been identified by knockdown of the gene B. minax in female adults, and B. minax flies significantly reduced their preference for green fruit after cutting Rh6 . Absence of a member of the bluesensitive opsin subfamily was found in both tephritid species C. capitata and B. minax, but Rh5 can be specifically expressed in Drosophila . Reports about vision-related genes directly involved in the host expansion of tephritids are still very few.Tephritid fruit fly hosts expand to other new host plants, and the phenology of the new host is another nonchemical stimulus that affects fly adaptation. The phenology of the host plant fruits includes the time of flowering, fruiting, or maturation .